This invention relates to a memory using a dynamic semiconductor memory.
A floppy disk is one of the conventional memory devices for file, but is not sufficiently reliable because it has a bulky mechanical portion. Moreover, it is not highly portable. Great hopes are therefore placed on semiconductor technology to develop electronized file memory devices. Since the semiconductor memories are volatile (or they lose the data stored therein when a power source is cut off), their application to the file requiring non-volatility is difficult.
Among the semiconductor memories, a dynamic memory, in particular, has power consumption which is incomparably greater than that of a static memory, and can not be used for the file application by means of battery back-up. Counter-measures for temporary breakdown of an external power source such as interruption of service are disclosed in Japanese Patent Laid-Open No. 153,580/1981, for example.
This prior art provides a dynamic memory module with a refresh circuit besides that of a CPU module, so that the memory is refreshed by the refresh circuit of the CPU module so long as an AC power source is supplied, and is refreshed by the refresh circuit of the memory module once the AC power source is cut off for some reason or other, thereby permitting the memory module to operate by itself. This construction eliminates the necessity that a back-up battery supplies the power to both CPU and memory modules, and the power consumption can be reduced because the power may be supplied to only the memory module.
However, the power consumption of dynamic memories must be reduced further drastically for those applications where the device is cut off from the external power source such as a file memory device for an extended period of time.